(i) Field of the Invention
This invention relates to a method for the production of weather-resistant boards and to the weather-resistant boards so produced.
(ii) Description of the Prior Art
Many wood fibre materials for the building industry are limited to interior use since they are not sufficiently weather resistant to be used externally. However, since these products, i.e., particle boards, fibre boards, insulating boards, plywood, etc. are considered to be ideal and inexpensive building materials because of their economy, their insulation properties and their lightness, the art has been faced with the problem of providing the necessary weather resistancy and aging stability by altering the nature and composition of the raw materials and binding agents as well as by applying a coating to the outer skin. The first aim not only is to prevent water from penetrating into the board, but also to keep off atmospheric humidity. Any attempts to solve this problem solely by the type of and modification of, the usual glue mixtures, and by adding hydrophobic agents and/or anti-bacterial agents, have not provided any reliable results, since it was also desired that the positive properties of the material, e.g. low costs, light weight, heat insulation, bursting strength and standard glueing, should be retained. Such properties are impaired when, for instance, too large quantities of binding materials or mineral admixtures are used, which were expedients recently attempted to achieve sufficient weather resistancy. Therefore, measures heretofore used to provide such weather resistance of the structural composition within limits are coupled with an effective surface protection.
Accordingly, up to the present time, neither materials nor methods have been found which would guarantee that the required long lasting protection would remain effective against atmospheric influences to maintain the board resistant to mechanical damage or destruction within an acceptable economic scope. In this respect, coating the board by liquid preserving agents is becoming more obsolete, since, the industry has been successful in developing products which have high resistance to moisture, UV radiation and heat radiation, etc. and which avoids to a large extent cracking or embrittlement. Another economic process has been provided in which synthetic-resin-soaked papers are pressed on during the standard production of boards or mouldings. This, however, results in thin and susceptible surface coatings which are liable to crack because of the brittleness of the material as soon as the base material expands. This danger would be overcome by laminating previously produced weather-resistant, impact-resistant, and crack resistant materials, e.g., sheet metals, asbestos cement, or synthetic foils to the board. These, however, might peel-off under the permanent strain of and influence of the weather as the differences of the expansion-coefficients due to heat radiation result in the destruction of the glue joint and cause blistering.
Accordingly, up to the present time, no process exists for use on wood-fibre materials, plywood materials or other fibre materials, even those which have previously been made moisture resistant to the greatest possible extent, which can be surface coated with an insoluble layer.